Magnetoresistance And A Crossover From Activated To Diffusive Dissipation In The Mixed State For Yba2cu3o7-Delta Epitaxial Thin Films

We measure the magnetoresistance of YBa2Cu3O7-delta epitaxial thin films as a function of angle theta between applied magnetic field H and the ab planes at a fixed temperature of 90 K below T-c, and a temperature T for Hparallel toc axis, respectively. The magnetoresistance curves for various angles theta show a three-dimensional anisotropic scaling behavior. The magnetoresistance curves reveal three distinguishable regions: a low-field nonlinear region, a high-field nonlinear one, and a linear one between them. The transition from a low-field nonlinear magnetoresistance to linear magnetoresistance shows a crossover from thermally activated to diffusive vortex motions as distinct from reported early. The H-T phase diagram of a vortex system is discussed.